Method for the construction of ferrite memory stores utilizing electrophoretic deposition



United States Patent 3,408,279 METHOD FOR THE CONSTRUCTIQN 0F FERRETE MEMORY STGRES UTILIZING ELEQTRGPHS- RETIC DEPGSITION Anthony Howard Collins, Chislehurst, Kent, England,

assignor to National Research Development Corporation, London, England, a British corporation No Drawing. Fiied Apr. 8, 1965, Ser. No. 446,725 Claims priority, application Great Britain, Apr. 11, 1%64, 15,052/64; Mar. 25, 1965, 12,654/65 7 Ciaims. (Cl. 204181) This invention relates to the construction of ferrite memory stores.

Ferrite memory stores are employed in electronic computers and usually comprise a stack of planes each of which consists of a network of wires laid at right-angles to each other to form a grid. The wires which are usually termed X and Y wires have ferrite material surrounding both the X and Y wires together at the points of crossover. In addition to the X and Y wires, which constitute the write wires there are also read wires passing close to the points of crossover.

The usual method of constructing such planes is to thread the read and write wires by hand through ferrite torroids. This method is time consuming and tedious.

According to the present invention a method of constructing a ferrite memory store comprises the steps of immersing a grid network of electrically conductive wires in a bath containing a liquid in which ferrite particles are suspended in such a manner as to prevent contact between wires at points of crossover of said wires in said network, applying a potential difference between said wires and an electrode immersed therein so as to cause electrophoretic deposition of said particles on the wires of said network until the deposits coalesce in the region of crossover of the wires in said network to form a continuous coating around each region of crossover, and sintering said deposits.

In order to ensure that the X and Y wires are held apart during the process of electrophoretic deposition they may be held in accurately constructed jigs while deposition takes place. Alternatively lengths of wire may be coated with a ferrite deposit by electrophoresis and the deposit partially sintered sufliciently to allow handling. A grid network is then constructed from the coated Wires and the network then completely sintered to cause the ferrite deposits in the region of crossover of the wires tocoalesce and to form a continuous coating of ferrite around each crossover. Also a further coating can be electrophoretically deposited after the grid network is constructed.

As a further alternative the wires may be first coated with a porous insulating material, for example crystalline alumina, before assembling into a grid network. In this way the porous insulation serves to separate the wires at the points of crossover and obviates the need for careful jigging. Furthermore the presence of porous insulating material between the wires inhibits the deposition of ferrite material between the wires at the points of crossover which may enhance the performance of the memory store.

To assist further in the prevention of deposition of ferrite materials between the wires at the points of crossover the wires may be formed as flat strips and assembled so that their fiat sides are facing one another at the points of crossover. The facing sides may be coated with nonporous insulating material at least in the region of crossover.

The liquid medium in which the network of wires is immersed preferably has a high electrical resistivity and may comprise alcohols, ketones, esters, hydrocarbons or pure water. The particles in suspension may comprise "ice either ferrite materials of the desired composition or else the materials from which a ferrite can be formed after deposition on the wires and sintering.

The wires themselves are preferably formed of a noble metal in order to withstand the sintering temperature. For example the wires can comprise platinum or a platinum alloy and it may also be desirable to match the coefficient of expansion of the wire with that of the ferrite to prevent the ferrite coating cracking due to the heating and cooling cycle.

An example of a suitable suspension of ferrite in a liquid medium contains the following constituents:

Square loop ferrite powder, e.g. a manganesemagnesium ferrite, presintered and ground For maximum green bulk density (that is to say, maximum bulk density in the unfired state) and minimal firing time, it is best to use mixture of ferrite particle sizes. For example, the particles may be constituted of 60% of particle size one micron and 40% of particle size 0.1 micron.

The ferrite material may be deposited at voltages from 6 volts to volts for one to two minutes to produce thicknesses of deposit in the green state of 0.001 inch to 0.005 inch or thicker deposits may be obtained if required. These thicknesses of deposit can be closely controlled by control of voltage and time.

In order to reduce strain in the ferrite deposit during and after sintering, an underlayer of graphite may be deposited on the assembly of wires before the deposition of the ferrite. This underlayer may then be oxidised away at a suitable stage in the firing process.

The sintering of the ferrite coating requires a firing schedule which is dependent on the nature of the ferrite material and on the green density of the deposit. For example, with deposits of low green density it is necessary to sinter the ferrite deposit for up to twenty hours at 1360 degrees Centigrade in order to reduce the porosity of the deposit. The sintering of the ferrite deposit is preferably carried out in a furnace having a controlled atmosphere in order that the composition of the fired ferrite can be controlled. An example of a firing schedule is as follows:

(1) Heat in air to 1360 C. for 20 hours.

(2) Allow to cool to 1100 C.

(3) At this point change the atmosphere from air to nitrogen to stop oxidation and maintain the temperature at 1100 C. for 2 hours.

(4) Allow to cool to ambient temperature.

Alternatively the sintering process may be carried out by heating the wires by passing current through them in a controlled atmosphere. In this case, the jig may be constructed of metals or other materials having a relatively poor temperature resistance. Otherwise the jig must withstand the heating cycle required for sintering the ferrite material and must be made from a refractory substance for example a ceramic such as pyrophyll-ite.

I claim:

1. The method of constructing a ferrite memory store comprising the steps of immersing a grid network of electrically conductive wires in a bath containing a liquid in which ferrite particles are suspended in such a manner as to prevent contact between wires at points of crossover of said wires in said network, applying a potential difference between said wires and an electrode immersed therein so as to cause electrophoretic deposition of said particles on the wires of said network until the deposits coalesce in the region of crossover of the wires in said network to form a continuous coating around each region of crossover, and sintering said deposits.

2. The method as claimed in claim 1 in which the wires are initially coated with a porous insulating material which serves to separate the wires in the region of crossover.

3. The method as claimed in claim 1 in which the wires are in the form of flat strips.

4. A method of constructing a ferrite memory store comprising the steps of assembling lengths of electrically conducting wire in a jig to form a grid network with the wires in the region of crossover being held apart immersing said grid network in a bath containing a liquid in which ferrite particles are suspended, applying a potential ditference between said network and an electrode immersed therein so as to cause electrophoretic deposition of said particles on the wires of said network until the deposits coalesce in the region of crossover of the wires to form a continuous coating around each region of crossover, and sintering said deposits.

5. The method as claimed in claim 4 in which an underlayer of graphite is deposited on the wires before deposition of the ferrite particles.

6. The method as claimed in claim 4 in which sintering is carried out by passing electric current through the wires.

7. The method of constructing a ferrite memory store comprising the steps of immersing lengths of electrically conducting wires in a bath containing a liquidtin which ferrite particles are suspended, applying a potential ditference between said wires and an electrode immersed therein so as to cause electrophoretic deposition of said particles on said wires, partially sintering said deposits, forming a grid network of said wires, and finally completing the sintering of said deposits to coalesce the deposits on wires in the region of crossover of said wires and form a continuous coating of ferrite around each crossover.

References Cited UNITED STATES PATENTS 2,906,682 9/1959 Fahnoe et a1. 204-181 3,031,386 4/1962 Tsu et a1 204-43 3,047,423 7/1962 Eggenberger et al. 204-43 3,099,874 8/ 1963 Schweizerhof 340174 3,100,295 8/1963 Schweizerhof 340174 JOHN H. MACK, Primary Examiner.

E. ZAGARELLA, Assistant Examiner. 

1. THE METHOD OF CONSTRUCTING A FERRITE MEMORY STORE COMPRISING THE STEPS OF IMMERSING A GRID NETWORK OF ELECTRICALLY CONDUCTIVE WIRES IN A BATH CONTAINING A LIQUID IN WHICH FERRITE PARTICLES ARE SUSPENDED IN SUCH A MANNER AS TO PREVENT CONTACT BETWEEN WIRES AT POINTS OF CROSSOVER OF SAID WIRES IN SAID NETWORK, APPLYING A POTENTIAL DIFFERENCE BETWEEN SAID WIRES AND AN ELECTRODE IMMERSED THEREIN SO AS TO CAUSE ELECTROPHORETIC DEPOSITION OF SAID PARTICLES ON THE WIRES OF SAID NETWORK UNTIL THE DEPOSITS COALESCE IN THE REGION OF CROSSOVER OF THE WIRES IN SAID NETWORK TO FORM A CONTINUOUS COATING AROUND EACH REGION OF CROSSOVER, AND SINTERING SAID DEPOSITS. 